Not which fan or is loud or why it is?
I made a desktop computer recently. I'm currently using an AMD-FX8150 processor, 16gb or DDR3 Ram(Geil EVO I believe), a MSI R7970 Lightning Radeon HD 7970 3GB graphics card, a pathetic 320 GB SATA HDD hard drive, a 700 Watt power supply, and an ASUS M5A97 Motherboard. I have a case fan in my case as well, but I know that one is not causing problems(tested it on its own).
What I'm wondering is which of my other 3 fans are being loud (cpu fan, GPU fans, and PSU fan). Another piece of information is that I'm using the stock CPU fan. It is not something that is ruining my computer, but it is kind of annoying. Also, my computer is not overheating so I know they are not overworked.
easiest way to check is set it on a table near ear height, take off the side panel and listen. It is usually not hard to tell whether the CPU or GPU is loud (rarely the PSU unless it is faulty fan or really old). both have small fans, smaller means more rpm to move the required amount of air, means louder. CPU fans/heatsinks tend to be more affordable to upgrade. If it is the CPU, a Cooler maser or zalman is usually great for the price.
Let me know if it doesn't work.